They did that some time ago, I seem to remember. The problem is identifying which ones are paid.

They would have a hard time identifying which author reviews are dishonest as well and their solution is to remove author reviews.

Authors: Amazon can't tell which reviews, if any are dishonest so they remove all.Fans: Amazon decides to remove their reviews, and when questioned about the decision threaten to remove the book.Paid reviewers: Amazon can't tell which reviews have been paid for, but know that some definitely were, so they decide to remove none.

Amazon decision benefits paid reviewers, because at this point only the reviews written by paid reviewers are safe.